1. Field of the invention
The present invention relates to an internal planetary gear mechanism used in a reduction gear or an overdrive gear, and in particular, to the amount of eccentricity α of an internal gear.
2. Description of the Related Art
The amount of eccentricity α of an internal gear in a conventional internal planetary gear mechanism is set to a theoretical value expressed by: (module)×0.5×(difference in number of teeth between the internal gear and an external gear).
More specifically, the theoretical value is expressed by:(φD1/N)×0.5×(M−N)where M is the number of teeth of the internal gear, N is the number of teeth of the external gear, and φD1 is a diameter of a pitch circle of the external gear. The amount of eccentricity α is conventionally set to the thus obtained theoretical value (for example, see Japanese Patent Laid-Open Publication No. Hei 7-243486).
(a′) During the operation of the internal gear and the external gear, the center of the external gear acts so as to oscillate with respect to the internal gear. As a result, the external gear acts so as to be pushed in the oscillating direction.
Since a rotational torque is expressed by: (turning radius)×(force), a force in the oscillating direction can be reduced more as a distance between the center of the external gear and the center of the internal gear is increased. As a result, a load applied to a bearing, which is provided between an eccentric part and the external gear, and a load (surface pressure) generated on meshing tooth surfaces of the internal gear and the external gear, can be reduced.
Since a loss can be expressed by: (coefficient of friction)×(load)×(velocity), the loss can be reduced by decreasing the load. However, since the amount of eccentricity α is set to the theoretical value in a conventional internal planetary gear mechanism, it is impossible to increase the distance between the center of the external gear and the center of the internal gear to reduce the load on the bearing or the tooth surfaces of the gears without increasing the size of the internal planetary gear mechanism.
(b′) The conventional internal planetary gear mechanism, in which the amount of eccentricity α is set to the theoretical value, has a large mesh zone between the internal gear and the external gear, thereby transmitting a turning force with a large number of tooth surfaces. Therefore, a mesh angle (pressure angle) is increased when coming closer to the ends of the mesh zone.
The turning force with respect to the force applied to the gears can be obtained by the following expression:(force applied to the gears)×cos(pressure angle)More specifically, as the pressure angle increases, a larger amount of the applied force becomes a loss instead of being a turning force. Thus, since the conventional internal planetary gear mechanism has a large mesh zone and thus has a large mesh part at a large pressure angle, a transmission loss of the turning force is increased.
(c′) The internal gear and the external gear are in contact with each other in such a way that their mesh points shift while they are sliding within the mesh zone during the operation.
However, since the conventional internal planetary gear mechanism has a large mesh zone and thus has a large slide contact area, a loss due to slide contact disadvantageously increases. More specifically, the number of positions where a loss is generated by slide contact is large to increase a transmission loss of the turning force.
Moreover, since slide contact occurs while a large load is being applied on the tooth surfaces of the respective gears, a large abrasion is caused on the tooth surfaces on which the slide contact occurs. In order to cope with this problem, the gears are quenched, which induces, however, a strain in the gears.